As far as sealing members, such as roller bearing seals and oil seals, which are used in sealed bearings and the like to prevent the rotary shaft lubricating oil from leaking and for the like purposes are concerned, various contrivances have been made for their sealing property which is their primary function, and for maintaining its property. To roller bearing seals and oil seals for rotary shafts to be used in automotive components, household electrical appliance parts and the like, heat resistance enabling long-term service as well as durability and oil resistance at the surface of contact is essential in order that the sealing property may be maintained for a long time.
The roller bearing seals and oil seals which are used in the automotive engine and related equipment, e.g. the transmission, axles, etc., are generally maintained at 120 to 130.degree. C. while the automobiles are running. However, when the car stops because of a traffic congestion, for instance, the effect of air cooling diminishes, allowing the seal temperature to exceed 150.degree. C. at intervals. Therefore, heat resistance is an important requisite for assuring the mechanical strength. The water pump bearing seal, alternator bearing seal, wheel system seal, etc. are liable to be exposed to rain water (in some instances, salt water and muddy water), radiator water (LLC-containing water), etc., hence sufficient water resistance is essential.
In the case of bearing seals and oil seals to be used in the automotive transmission, transaxle, differentials and the like, not only functionally sufficient mechanical strength but also high heat resistance is required in order that they may serve the purpose continuously in the lubricating oil environment of 130 to 140.degree. C. at the maximum. Furthermore, a contrivance is required for preventing contaminants in the lubricating oil from entering the bearings and further for reducing seal lip abrasion due to the contaminants. Thus, in addition to said mechanical strength and heat resistance, good creep resistance (CS) is essential.
In the case of sealing members to be used in automotive power steerings and the like, still higher mechanical strength is essential for resisting the high internal pressure, in addition to the above-mentioned heat resistance.
In the case of bearing seals and oil seals to be used in the automotive crankshaft, cam shaft and the like, a low-aniline-point performance is essential for resisting the engine oil or fuel oil which has the property to swell rubber, in addition to the above-mentioned heat resistance.
In the case of the bearing seal or oil seal for the compressor as a component of the automotive air conditioner, household or office air conditioner, refrigerator or the like, those conventional machine oils for compressor use which are based on mineral oils are being switched over to a synthetic lubricating oil polyalkylene glycol (PAG), from the viewpoint of compatibility with a substitute fluorocarbon, which has recently been used as a heating medium in lieu of the fluorocarbon which tends to destroy the ozone layer. Thus, Japanese Kokai Publication Hei-3-160186 discloses a technology of improving compressors using PAG as the lubricating oil and Japanese Kokai Publication Hei-3-281688 discloses a technology of using PAG as a synthetic lubricating oil in the refrigerating machine. PAG may gasify (as a volatilized oil or a mist) during use. In that case, the gasified PAG and air come into simultaneous contact with the bearing seals or oil seals, so that it is essential that these have sufficient PAG resistance.
Meanwhile, nitrile rubbers (NBRs) having good oil resistance have so far been used as the material for the above-mentioned bearing seals or oil seals. However, NBRs fail to satisfy the heat resistance requirement. For instance, they are not suited for use in the vicinity of an engine where the temperature may rise to about 150.degree. C. The use of hydrogenated NBRs (HNBRs) as alternatives was considered. However, they are expensive, hence the use thereof presents a problem from the economic viewpoint.
Therefore, the use of acrylic rubbers (ACMs) or fluoro rubbers (FKMs) has been proposed. However, ACMs are disadvantageous from mechanical strength and oil resistance viewpoints and FKMs have drawbacks in that they are expensive and, hence, uneconomical.
A technology of incorporating a certain amount of a (meth)acrylate in butadiene and acrylonitrile which are components of the NBRs, to give modifiled nitrile rubbers with improved processability is disclosed in U.S. Pat. No. 2,395,017. Japanese Kokoku Publication Sho-47-21579 discloses a technology of copolymerizing butadiene, acrylonitrile, chloroacrylonitrile and an alkyl acrylate to give a modified nitrile rubber with improved heat resistance. Japanese Kokoku Publication Sho-60-11744 discloses a technology of copolymerizing a conjugated diene, an unsaturated nitrile and an unsaturated dicarboxylic acid ester and incorporating a specific additive in the copolymer to provide the same with heat resistance.
In Japanese Kokoku Publication Sho-62-59138, there is disclosed a technology of incorporating a vulcanization agent and a crosslinking agent in a modified nitrile rubber with a limited Mooney viscosity as obtained by specifying the proportions of the constituents, namely butadiene, acrylonitrile and a hydroxyalkyl (meth)acrylate, to thereby insure processability.
In the case of sealed roller bearings, which support rotating shafts in compressors, too, attention has so far been paid only to the use, as the material for sealing plates, of a material excellent from the lubrication and heat resistance viewpoints. Thus, from the cost viewpoint as well, the use of acrylic rubber materials and the like, for instance, has been preferred.
The grease so far used as a lubrication medium in bearings is composed of a poly-.alpha.-olefin base oil and, as a thickening agent, diurea. Therefore, EPDMs or chlorinated polyethylene (CM), though resistant to PAGs, has not been regarded as being suited as the sealing plate material mentioned above.
When a PAG is used as the lubricating oil in a compressor, for example in a swash type rotary compressor (FIG. 2), the lubricating oil PAG sealed in a cavity 11 of a sealed vessel 10 contributes to lubrication of the swash plate during rotation. However, at high temperatures encountered, for instance, when the compressor is used in an automotive air conditioner, the PAG is gasified and the gas goes through the clearance within the vessel and arrives at the roller bearing 12 on the rotary shaft 13. Once the PAG gas has arrived at the roller bearing 2 (FIG. 1), the sealing plates 5 and 6 which seal the roller bearing 2 are in contact with air at all times, and the sealing plates 5 and 6 come into a state of simultaneous contact with air and PAG gas.
Under such circumstances, due to simultaneous contact of the sealing plates with air and PAG gas, the sealing plates made of an acrylic rubber, which has previously been used for a urea-type grease, have presented problems, namely corrosion and deterioration.
To solve the above problems, an attempt was made to use nitrile rubbers (NBRs), which have a PAG-resistant characteristic, as materials for the sealing plates mentioned above. NBRs are also resistant to mineral oil-based greases, hence are suited for that purpose. However, their heat resistance, as determined in a heat aging test in air, is at most about 125.degree. C. Thus, there is a problem that NBRs cannot be used in automotive air conditioners where a heat resistance of 130 to 150.degree. C. is required.
It is conceivable to use hydrogenated NBRs in lieu of NBRs. However, though they solve the heat resistance problem, they raise the cost markedly, hence they can hardly be put to practical use.